SMC Networks SMC2804WBR V.2 - Manual (Page 65)

Advanced Setup
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Special Applications
Some applications, such as Internet gaming, videoconferencing,
Internet telephony and others, require multiple connections.
These applications cannot work with Network Address Translation
(NAT) enabled. If you need to run applications that require
multiple connections, use the following screen to specify
the additional public ports to be opened for each application.
Specify the public port number normally associated with an
application in the Trigger Port field. Set the protocol type to
TCP or UDP, then enter the ports that the application requires.